1. Field of the Invention
This invention relates to holographic techniques for fabricating high spatial frequency thin film wire grid polarizers. In particular, this is for thin film wire grid polarizers in the near infrared region of the spectrum.
2. Description of the Prior Art
Conventional wire grid polarizers are used for operation from 1 to 1000 .mu.m. In the 20 to 1000 .mu.m range, the grids are produced by contact printing a grating mask onto a photoresist layer covering an aluminized plastic substrate, followed by chemical etching. Etchant undercuttings set an upper limit to the grid's spatial frequency. This limits use to wavelengths longer than 20 .mu.m.
For the 1 to 30 .mu.m range, two different methods are currently in use. The first technique is a blaze diffraction grating which is mechanically ruled on a highly polished substrate such as ZnSe. This is followed by vacuum deposition of aluminum at an oblique angle to form the conductive wires at the groove peaks. Wire grid polarizers produced by this method are limited by the ruling machine resolution to use at wavelengths longer than about 2 .mu.m. This method is relatively expensive due to the precision machining and polishing required.
In the second technique, wire grid polarizers are fabricated by first holographically recording a grating in photoresist. This is accomplished by recording a two-beam laser interference pattern. The chemically developed photoresist surface's relief grating is then illuminated at an oblique angle to produce the conductive grid pattern. This method is expensive and resolution limited to wavelengths longer than 1 .mu.m.
Resolution limitations inherent in these conventional fabrication techniques limit their use to wavelengths longer than about 2 .mu.m if high polarization is required. Chemical etching techniques are limited to spatial frequencies less than 1000 l/mm, limiting use to wavelengths greater than 20 .mu.m. Machine ruling methods can yield 1000 to 1500 l/mm for application at wavelengths longer than 2 to 3 .mu.m. Aluminized holographically recorded wire grids are capable of fabricating grid patterns of 2000 to 4000 l/mm corresponding to usage at wavelengths of 1 to 2 .mu.m.
Thus it can be seen that a technology void exists to fill the need for high efficiency polarizers below the 1.0 .mu.m spectral region. Polarizers currently used in this region are polaroid-thin film polarizers. They suffer from an increase in unpolarized light transmittance which reduces obtainable dynamic range. An increase in unpolarized light transmittance of greater than 10% is sufficient to defeat current techniques.
It is an object of the present invention to produce wire grids which extend the useful wavelength range below the 1 .mu.m cutoff of conventionally produced high efficiency grids. It is another object of the present invention to produce polarizers for the infrared region corresponding to the 1.0 .mu.m to 1000 .mu.m wavelength range.
It is a further object of the present invention to describe a technology capable of producing wire grids of better than 4300 l/mm with variable wire width.
This and other objects of the present invention will become clear upon further reading of the present invention.